A Novel Approach for Identifying and Assessing MOR-Based CMIP6 Model for Hydrological Analysis in an Ungauged Watershed

The identification of the onset and retreat dates of the monsoon season is a crucial and intricate phenomenon, given its annual spatiotemporal variability. The monsoon season contributes significantly to rainfall, replenishing water sources and hydrological systems but causes hydrological extremes, especially for the high-altitude watersheds in Southeast Asia. Global Circulation Model (GCM)-Coupled Model Intercomparison Project Phase 6 (CMIP6)-based rainfall and temperature data are helpful for adequately representing present and future climate scenarios. However, the usability of uncorrected GCM-CMIP6 datasets needs to be assessed regionally. This study focuses on identifying the best-suited GCM-CMIP6 based on the monsoon onset (M_O) and retreat (M_R) dates along with other climatological temporal parameters. A numerical definition for M_O and M_R has been formulated to find the best-suited GCM-CMIP6 (i.e., CMCC-ESM2). In this context, runoff simulation is carried out using the meteorological inputs of the monsoon onset-retreat (MOR)-based best-suited GCM to evaluate its usability. A multi-model simulation approach has been carried out for runoff estimation based on observed datasets to find a better-suited hydrological model. The proposed overall methodology is tested in a hydrological extreme-prone ungauged watershed (i.e., Ranikhola). CMCC-ESM2 and SSP2-4.5 has been identified as best-suited SSP based on statistical evolution (R² [0.693], NSE [0.662] and RSR [0.581]) for future daily runoff prediction. Future hydrological analysis shows that the average monsoon peak runoff magnitude will increase from the calibrated period (2015–2020) by 18.01% in the coming years (2021–2049).